Mobile communications systems using a frequency band of hundreds of megahertz to several gigahertz have terminal devices comprising a receiving circuit 91 and a transmitting circuit 92 which are connected in parallel with an antenna 9 via a duplexer 40 to use the single antenna 9 for both receiving and transmitting signals as shown in FIG. 15. The duplexer 40 comprises a receiving filter 1 and a transmitting filter 2, each of which can be provided, for example, by connecting a plurality of coaxial dielectric resonators 50 as shown, for example, in FIG. 16(a).
With reference to FIG. 16(a), the coaxial dielectric resonator 50 comprises a rectangular parallelepipedal dielectric block 51 having a bore extending therethrough, an outer conductor layer 53 and an inner conductor layer 52 which are formed on the dielectric block 51 respectively over the outer peripheral surface thereof and the inner peripheral surface thereof defining the bore, and a short-circuiting conductor layer 54 formed on the dielectric block 51 over an end face thereof where the bore has an opening and providing a short circuit between the outer conductor layer 53 and the inner conductor layer 52.
With the coaxial dielectric resonator 50, the outer conductor layer 53 is connected to the ground, and the inner conductor layer 52 to a signal input terminal S, whereby the coaxial dielectric resonator 50 is made equivalent to a circuit comprising an inductance element L and a capacitance element C which are connected in parallel with each other as shown in FIG. 16(b), thus providing a trap filter having a resonance frequency which is determined by the inductance of the inductance element L and the capacitance of the capacitance element C.
A dielectric duplexer is also made available which comprises a plurality of coaxial dielectric resonators 50 assembled into a single block and providing a receiving filter 1 and a transmitting filter 2 (see U.S. Pat. No. 5,250,916). For example, FIG. 9 shows a dielectric duplexer 40 comprising a plurality of dielectric resonator portions 7a to 7g which are integrally assembled into a common dielectric block 11.
With reference to FIG. 10, the coaxial dielectric resonator portions 7 have a dielectric block 41, respective inner conductor layers 43 formed on the dielectric block 41 over inner peripheral surfaces thereof defining bores 42 extending through the block 41, an outer conductor layer 44 formed over the outer peripheral surface of the block 41 and a short-circuiting conductor layer 45 providing a short circuit between the outer conductor layer 44 and the inner conductor layers 43.
A conductor pattern 46 is formed on the dielectric block 41 over one end face thereof where the bores of the resonator portions 7 each have an opening, for connecting the resonator portions to one another. As shown in FIG. 9, the conductor pattern 46 comprises a plurality of conductor pattern pieces 21 to 27 provided for the respective resonator portions 7 and each extending from the opening edge around the bore outwardly thereof, a conductor pattern piece 47 connectable to an antenna, a conductor pattern piece 28 connectable to a receiving circuit, and a conductor pattern piece 29 connectable to a transmitting circuit.
FIG. 11 shows a circuit equivalent to the dielectric duplexer 40. As illustrated, the receiving filter 1 and the transmitting filter 2 are connected in parallel with an antenna connecting terminal portion ANT. The receiving filter 1 comprises a filter circuit 3 having a pass band in the frequency band of the signals to be received, and a trap circuit 4 for suppressing the frequency band of the signals to be transmitted. The transmitting filter 2 comprises a filter circuit 5 having a pass band in the frequency band of the signals to be transmitted, and a trap circuit 6 for suppressing the frequency band of the signals to be received.
A connecting terminal Rx for the receiving side is provided at a connection between the filter circuit 3 of the receiving filter 1 and the trap circuit 4 thereof. A connecting terminal Tx for the transmitting side is provided at a connection between the filter circuit 5 of the transmitting filter 2 and the trap circuit 6 thereof.
The filter circuit 3 of the receiving filter 1 comprises three coaxial dielectric resonator portions 7a to 7c, and the trap circuit 4 comprising one coaxial dielectric resonator portion 7f. Further the filter circuit 5 of the transmitting filter 2 comprises two coaxial dielectric resonator portions 7d and 7e, and the trap circuit 6 comprising one coaxial dielectric resonator portion 7g. Incidentally, each of the coaxial dielectric resonator portions 7 has the same construction as the coaxial dielectric resonator 50 shown in FIG. 16(a).
With reference to the circuit shown in FIG. 11 and equivalent to the dielectric duplexer 40, a signal line extending from the antenna connecting terminal portion ANT is provided with nine capacitance elements C1 to C9, which are provided by the conductor pattern 46 formed on one end face of the dielectric block 41 of the duplexer 40 shown in FIG. 9. Stated more specifically, the pair of coaxial dielectric resonator portions 7, 7 to be connected to each other have their conductor pattern pieces positioned close to each other to provide a capacitance between the two conductor pattern pieces.
Cellular phones or portable telephones developed in recent years have various additional functions such as GPS function and radio LAN function. Such portable telephones have a transmitting circuit and a receiving circuit, each of which is provided with a trap circuit for suppressing the operating band of the additional function. However, provision of the trap circuit for each of the transmitting circuit and the receiving circuit entails the problem of necessitating a circuit board of increased size.